1. Field of the Invention
The invention relates to a process for producing epitaxially coated semiconductor wafers from silicon, known as epi wafers. Specifically, the invention relates to the production of p−-doped and epitaxially coated semiconductor wafers from silicon, which are distinguished by a high getter capacity and by a low number of layer defects and are therefore eminently suitable for further processing to form large scale integrated electronic components.
2. Description of the Related Art
Unlike the case of heavily boron-doped, p+-doped semiconductor wafers, the ability to keep metallic impurities away from the electrically active region is inadequate in the case of lightly boron-doped, p−-doped semiconductor wafers. In general, such wafers form insufficient oxygen precipitates (BMDs, bulk micro defects) which ordinarily bond such impurities and in this way act as what are known as intrinsic getters. In the case of lightly boron-doped semiconductor wafers made from silicon, nucleation centers for oxygen precipitates are destroyed to such an extent during epitaxial coating that the resulting p−-doped epi wafers no longer have a sufficient getter capacity, which is assumed to require a density of at least 5×108 cm−3 oxygen precipitates.
It is known that the formation of nucleation centers for oxygen precipitates and ultimately also the getter capacity can be increased if the semiconductor wafers are additionally doped with nitrogen and/or carbon. The nitrogen added by doping has the effect of making the nucleation centers more stable at higher temperatures, including the temperatures used during epitaxial coating. On the other hand, however, nitrogen also promotes the formation of oxidation-induced stacking faults (OSFs) in a region of the semiconductor wafer known as the OSF ring. Consequently, increased levels of layer defects also occur in this region on the surface of an epitaxially deposited layer, which can have a highly adverse effect on the functionality of electronic components and should therefore be avoided as far as possible. Layer defects (LPDs, light point defects) of this type are rendered visible by optical methods using scattered laser light.
It is proposed in EP 1 143 045 A1 to restrict the nitrogen concentration to a low range of from 2×1013 cm−3 to 1×1014 cm−3 and to carry out epitaxial coating only on semiconductor wafers made from silicon which, with regard to point defects, have a void-rich region (v-region) with or without an OSF ring over the entire wafer surface. This method has the drawback that the required boundary conditions entail increased outlay in terms of monitoring and time when pulling the single crystal. For example, the cooling rate has to be kept low in the range from 1000 to 900° C., in order to alter the morphology of the OSF nuclei. A low cooling rate generally requires the single crystal to be pulled at a low and therefore uneconomical pulling rate. A further particular drawback is that the segregation-related rise in nitrogen concentration is at odds with the efforts to limit the nitrogen concentration. The nitrogen concentration rises greatly as the length of the single crystal increases, and consequently can quickly move out of the restricted range. Layer defects may then still be avoidable if the oxygen concentration is kept at a low level. However, this likewise requires increased monitoring outlay and in turn gives rise to the problem that it is possible that insufficient oxygen may be present to form sufficient numbers of nucleation centers for oxygen precipitates.